1. Field of the Invention
The present invention relates to a non-linear optical device which modulates or switches a light and carries out an operation by the light by making use of an optical non-linearity of a material Such a device serves as a basic component in transmitting or processing information by using the light.
2. Related Background Art
A non-linear optical device (NOD) have been reported by G. I. Stegeman et al in the Journal of Lightwave Technology, 6,953 (1988). A typical example of the NOD introduced in the above article is shown in FIG. 1, in which two optical wave guides a and b are coupled in a coupling area c shown by hatching lines. When light inputs P1 and P2 are applied to input terminals (left side in FIG. 1) of the optical wave guides a and b, light outputs P3 and P4 are produced at output terminals (right side) A useful function may be attained by using a non-linear optical material (NOM) in the coupling area c. Namely, a ratio of P1 and P2 may be varied in accordance with intensities of P1 and P2. This makes use of the fact that the coupling length is determined by a refractive index of the area c and the refractive index depends on the light intensity by the optical nonlinearity. By making use of this, an optical AND gate or an optical transistor is fabricated. The NOM used in the coupling area c may be a dielectric material such as LiNbO.sub.3 or a semiconductor material such as GaAs, but the latter is superior from the standpoint point of integration with a light emitting device and machining work. In the present invention, the semiconductor conductor material is and therefore explained by comparing a prior art device which also uses the semiconductor material.
In the prior art device, the nonlinearity in a vicinity of a band end of a GaAs or GaAs/AlGaAs multi-quantum well structure (MQWS) was utilized. Namely, a light energy h.omega. of the light which activates the NOD is selected to be h.omega..apprxeq.Eex, where Eex is an energy generated by an exciter of the semiconductor, in order to attain a large nonlinearity. However, this raises the following problems.
(1) The light pulse is distorted in the vicinity of the band end because of a large dispersion of the (linear) refractive index. As a result, it is not possible to apply the very short pulse at a high repetition frequency to activate the NOD, and a very high speed operation which should be an advantage of the optical information processing is not attained.
(2) Because of large absorption, the light intensity is significantly attenuated each time the light passes through each NOD. As a result, frequent amplification in the course of transmission is required. Further, because the absorption .chi..sup.(n) /.alpha. (where .chi..sup.(n) is an n-order non-linear acceptability, and .alpha. is an absorption coefficient), which is a figure of merit for the NOM, does not increase even if .chi..sup.(n) is large, because .alpha. is large.
It has been known that the absorption can be reduced by setting h.omega. to be slightly smaller than Eex, but even in such a case, the absorption accompanied by phonon absorption which extends up to several tens of mV below Eex and the two-photon absorption in the range of h.omega..gtoreq.Eex/2 are not avoidable.